The invention relates to communication equipment for audio frequency signals, and especially equipment providing protection against high-level noise signals and other unwanted high-level signals.
The system according to the invention is developed particularly for use in connection with the applicant's U.S. Pat. No. 5,058,155, concerning a multipurpose headset amplifier; consequently, all which appears from the said USA patent by this reference is included in the present application.
The audio frequency signal compressing system according to the present application can be applied as an independent circuit for the protection of a headset user against unwanted high-level signals. However, the system also allows for integration and can form part of the multipurpose headset amplifier according to the above mentioned USA patent; it then replaces blocks 33 and 34 in FIG. 2 of the drawing.
Nowadays telephone systems are very extensively used, and not only for ordinary telephone communication, but also for communication by means of electronic equipment such as computer modems, telefax machines, automatic data transmission equipment, etc. As a result of this increased use of the telephone network, wanted signals as well as unwanted signals such as transients, impulses, continuous high-level sounds, etc., appear on the network lines.
When using an ordinary telephone apparatus provided with a handset to be held in the user's hand, the telephone receiver can quickly be removed from the user's ear if an unpleasant signal level appears, If, however, a headset is used, e.g. as known from the applicant's U.S. Pat. No. 4,893,344, or any corresponding headset to be mounted on the user's head, the user will not be able to quickly remove the receiver from his/her ear(s) and will thus involuntarily be affected by the high level signals. The extensive use of telephone headsets has aggravated this problem.
To properly protect against continuous noise, the U.S. Occupational Safety and Health Administration (OSHA) has established a 85 dBA(t) limit for 8 hours time weighted averaged measured signal on an A scale. The designation "dBA(t)" is 20 times the log of a sound level with respect to 20 micropascals, which is "A" weighted and time averaged.
To protect against high-level sound signals, AGC (Automatic Gain Control) circuitry in receiver amplifiers is normally adjusted so that the earphone speaker output will not exceed 94 dBSPL. The term "dBSPL" refers to "sound pressure level", which is the same as "dBA(t)" except that the "A" weighting curve is removed along with the time averaging. Limiting sound to a dBSPL rating is easily accomplished by the use of ACC circuitry with peak detecting control.
Within the difference between the "dBA(t)" and "dBSPL" sound ratings, there lies a problem. The "dBA(t)" limit allows for the acoustic peaks and valleys that normally occur in speech by virtue of its time averaging feature, However, the peak detection methods required by the "dBSPL" circuits do not. The crest factor of a continuous sine wave is 1.414, whereas the crest factor of normal speech may exceed 5 (the "crest factor" can simply be described as the ratio of a waveform's "peak" value to the "rms" value).
Because of the crest factor, AGC circuitry will limit human voice signals to a level far below 94 dBA(t). User safety with respect to peaks and continuous sound is preserved with this limit, but the level of human voice output signal from the earphone speaker in normal environments is difficult to understand. This has its own deleterious effect by adding user stress because of the strain required to hear the signal and by decreasing productivity due to the repeated questions and statements regarded during conversations.
The optimum solution is to limit voice and continuous signals equally to 85 dBA(t). However, the crest factor of the two signal types does not allow an easily integratable peak detecting limiter circuit to be utilized.
Prior art technique known from U.S. Pat. No. 4,928,307 for protection against continuous high-level signals includes a voice signal compression system wherein the compression threshold may be adjusted to a calibrated level. The system includes a variable gain amplifier for amplifying a voice input signal based on the application of a variable control voltage. A driver increases the power gain of the amplified input signal to provide a power-boosted output signal. In addition to applying the power-boosted output signal to an acoustic output transducer, the power-boosted output is also applied both to a peak detecting comparator and to a threshold reset timer. The comparator produces a digital output that triggers an attack/decay timing generator which produces the control voltage for the variable gain amplifier. The comparator initially is sat to a high threshold state by the threshold reset timer. An the levels and peaks of the input signal increase, the comparator begins to trigger digital pulses to the attack/decay timer. At the same time, the threshold roast timer begins measuring the duration of the signal which exceeds the threshold of the comparator. If the threshold reset timer detects that the level of the power-boosted output signal exceeds the threshold of the comparator for a preselected time, then the compression threshold of the comparator is switched to a lower level. The threshold remains low until the continuous signal is removed. At this point, the circuit in reset to its normal mode of operation.